This invention relates to the inhibition of ice formation at moderate supercooling temperatures and, more particularly, to the protection of light frost-sensitive plants against frost injury.
Damage to crops by frost is one of the leading causes of loss in agricultural output due to natural phenomenon variability in the world, to be exceeded only by drought and flooding, pests and disease. It is estimated that from 5-15% of the gross world agricultural product may be so lost to frost damage in one year. In some regional areas (i.e. counties, valleys) the loss may approach 100%.
The greatest amount of frost damage to sensitive crops does not occur in northern or cold climates. Instead, it occurs at mid- and low-latitudes and at high altitude equatorial locations where high value food crops such as soybean, corn, orchard fruits, and vegetables are grown. For instance, the orchards of California, vineyards of Italy, the corn and soybeans of Iowa, and potatoes of Ecuador all suffer damage each year from the same phenomenon--light night frost at temperatures from -1.degree. C. to -4.degree. C.
It has been estimated by the United States Department of Agriculture that about 1.5 billion dollars of agricultural products is lost to frost damage in the United States each year. The worldwide total is probably in excess of 10 billion dollars.
For the most part, present frost protection methods are centered around the principle of maintaining heat in a crop to keep it from cooling below the freezing point where frost is imminent. This is done by a variety of methods such as burning oil or natural gas, stirring the air over crops, sprinkling the crops with water, and covering them. With the cost of petroleum becoming more expensive and pressures against polluting the air with anthropogenic fires, heating large areas of agricultural land to prevent frost damage may become increasingly unpopular in the future. Also, these measures all require a considerable amount of equipment, trained and available manpower, and are capital intensive.
In addition to these physical methods, chemical methods of frost protection for growing plants have been attempted by application of various chemical compounds onto the plants with the view of lowering the temperature at which the plant tissues would freeze. These previously proposed chemical methods have tended to be unreliable, expensive, and ecologically unsound.
Frost damage to plants occurs when intracellular liquid in the plant tissues freezes with resulting rupture of adjacent cell walls and cell membranes. It is known that plant tissues may supercool to temperatures of around -6.degree. C. in the absence of external ice nuclei. The internal plant tissues do not generally initiate ice at temperatures warmer than this -6.degree. C. threshold.
It has recently been established that there are a very few bacteria species which can act as ice-forming nuclei at relatively warm temperatures, i.e., -1.degree. C. to -3.degree. C. The bacteria Erwinia herbicola and Pseudomonus syringae have been identified as being representative, if not the sole species, of these bacteria acting as ice nucleants on plant tissues.
To protect plants from frost damage, it is therefore desirable to have available means for reducing the populations or otherwise inhibiting the ice-nucleating activity of the ice-nucleating bacteria on plant leaves, so as to thereby reduce the temperature at which frost injury occurs to temperatures approaching -6.degree. C. The use of various chemical bactericides for this purpose has not thus far proven to be a satisfactory approach, since besides being expensive and ecologically unsound, such bactericides have not been species specific to the ice-nucleating bacteria, but instead have been deleterious to the plants by also killing the beneficial bacteria.
Another recently proposed approach to this problem, as described in the Arny et al U.S. Pat. Nos. 4,045,910 and 4,161,084, incorporated herein by reference, is to apply to the plants competitive non-ice-nucleating bacteria in an amount sufficient to increase the proportion of non-ice-nucleating bacteria to ice-nucleating bacteria from that normally present on the plants, thereby reducing the probability that sufficient numbers of ice-nucleating bacteria will be able to grow on the plant leaves. This approach requires application of the competitive bacteria at a rather substantial time prior to the onset of freezing temperature and/or at a rather early stage of plant growth so as to enable the competitive bacteria to adequately establish themselves on the plant leaves in order to be effective, and has not been found to be fully reliable or confidently repeatable in field trials.